1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device and a driving method thereof which are capable of preventing generation of a residual image upon power-off.
2. Discussion of the Related Art
Display devices are classified into an emissive display device such as a cathode ray tube, an organic electro-luminescence display device, or a plasma display device (PDP), which can emit light by itself, and a non-emissive display device such as a liquid crystal display (LCD) device, which cannot generate light by itself, so that it requires a separate light source.
A general LCD device includes two display panels provided with electric field generating electrodes, and a liquid crystal layer interposed between the display panels. The liquid crystal layer has dielectric anisotropy. In the LCD device, an electric field is generated at the liquid crystal layer as a voltage is applied to the electric field generating electrodes. The intensity of the electric field is adjusted as the voltage is varied. In accordance with the adjustment of the electric field intensity, the transmittance of light passing through the liquid crystal layer is controlled. Thus, a desired image is obtained. The light may be light generated from a separate artificial light source provided at the LCD device, or may be natural light.
For the light source of the LCD device, several lamps are typically used. In particular, for a light source capable of uniformly supplying light to the overall portion of the liquid crystal panel at the back surface of the liquid crystal panel, a fluorescent lamp such as an external electrode fluorescent lamp (EEFL) or a cold cathode fluorescent lamp (CCFL), or a light emitting diode (LED) is used.
Since the LCD device, which is a non-emissive display device, displays an image, using light emitted from a backlight, the display quality of the LCD device is determined in accordance with the luminance of the backlight. However, the backlight source has a problem in that a luminance deviation is exhibited due to ambient temperature, the heat internally generated from the display device, and non-uniform characteristics of the backlight source. Such a luminance deviation, which may occur in all light sources, causes a degradation in the display quality of the LCD device.
In particular, the most excellent advantage of an LED backlight for an LCD device, which is being actively researched and developed, is that light beams respectively emitted from red, green, and blue light sources are supplied in a mixed state to the LCD device. However, the LED used for a backlight source exhibits an abrupt light efficiency variation due to heat. In other words, the LED responds sensitively to the surrounding environment of the LCD device or the internal heat source of the LCD device, thereby causing the balance of colors to collapse.
In other words, each light source is gradually degraded as the driving time thereof increases, so that the drivability thereof is degraded. That is, although a drive signal having an initially-set duty rate is always supplied, the light source cannot provide an original luminance. In order to prevent this phenomenon, the backlight driver increases the duty rate of the drive signal so that the light sources can output a normal luminance. However, it is difficult to compensate for the luminance of the light sources, only through an increase in duty rate, when the luminance of the light sources decreases to about 50% of the original luminance. This is because the drive signal has a limited margin period. If any one of the three-color light sources cannot generate a normal luminance due to the above-mentioned reason, even though the remaining two light sources generate the normal luminance, the luminance of the color-mixed light is degraded. Furthermore, the color of the mixed light is different from a target color.